Two boatloads of asylum seekers are believed to be aboard Australian vessels off Christmas Island but the government has refused to confirm their existence or comment on whether they will be allowed to set foot in Australia.

By Sunday evening it had been more than 24 hours since any civilian communication with a former fishing trawler carrying 153 Tamil asylum seekers, including 37 children, that left the southern Indian city of Pondicherry 16 days ago.

Refugee advocates believe they have been intercepted by the navy and transferred to the customs vessel Ocean Protector, which is equipped to hold 250 people.

A second boat that left Indonesia is also believed to have been stopped off Christmas Island by the customs ship Triton.

If the asylum seekers are taken to Christmas Island before being flown to Nauru or Manus Island, it will snap a six-month period during which no boats arrived in Australia under the Coalition policy of boat turnbacks and offshore processing.

West Australian Labor MP Alannah MacTiernan, who was on Christmas Island over the weekend, said: ''Everyone is on standby waiting for instructions as to whether or not the boat is going to be unloaded here or whether or not it's going to be taken elsewhere.''

A spokesman for Immigration Minister Scott Morrison declined to provide any information.

Greens senator Sarah Hanson-Young said the more than 30 children believed to be on board the Tamil boat, including a one-year-old baby, should not be kept on a customs vessel.

''It is more like operation prison ships than it is Operation Sovereign Borders,'' she said. ''I'm extremely concerned that the government is holding these asylum seekers on board customs vessels. We know they've been doing that in the past with other boats for a number of days, weeks. A prison ship is no place for children.''

Refugee Action Coalition spokesman Ian Rintoul said by 5.30pm on Sunday it had been 26 hours since phone contact was lost with the Tamil boat when it was 175 nautical miles off Christmas Island.

Its occupants had reported an oil leak, a diesel shortage and sickness among the passengers.

Gladston Xavier, a refugee advocate in camps in southern India, said reports of the boat's distress had reached refugee communities there.

''There are people missing from the camps, and their relatives are asking about what has happened to their loved ones,'' he said.

"People living here are vulnerable and some had been preparing for quite a long time to take this dangerous journey to Australia.

''People here know about the dangers, and they know about the terrible laws in Australia, which mean you cannot get residency but, still, a small number of people have made the decision to go."

Dr Xavier said if a boat was in distress in Australian waters, the country had a legal, as well as moral, obligation to offer help.

"Humanitarian considerations need to be the highest priority. If this boat is in distress, authorities must give assistance, they cannot let these people die on the high seas. People here are very scared for the suffering of the people on the boat, for their lives."

More than 70,000 Sri Lankan Tamil refugees live in isolated refugee camps in the state of Tamil Nadu, in southern India. They are restricted from moving or working, and cannot apply for permanent visas or citizenship.